This invention relates to apparatus for weighing collected fluid during the collection process and, more particularly, to means for isolating the weighing mechanism from coupling forces of the fluid collection system and a method of using same to obtain improved accuracy in weight measurements.
Various apparatus are known for determining the urinary output of a patient which is measured in units of volume. In U.S. Pat. No. 4,448,207 of Parrish, the height of fluid in a collection container is determined through ultrasonic echo sensing, and then the volume is calculated by a computer based upon a stored volumetric profile of the collection container and the determined height. Similarly, in U.S. Pat. No. 4,051,431 of Wurster, the level of a fluid in a container is determined by means of capacitive sensing from which the volume can be determined in a rigid container. In U.S. Pat. No. 4,314,484 of Bowman, an optical drop count apparatus first forms drops from the collected fluid of approximately a known uniform volume and then counts the drops to determine the total volume accumulated. A similar apparatus is also shown in U.S. Pat. No. 4,291,692 of Bowman et al., 4,347,316 of Jesperson and 4,286,590 of Murase.
Other techniques of measurement are also known. In U.S. Pat. No. 4,417,585 of Franck, moveable measurement containers are alternately filled and dumped in a collection container, and the volume is calculated by measuring the number of fill and dump cycles. Similarly, in U.S. Pat. No. 4,206,767 of Winegrove, a plurality of measurement tubes are successively filled and sensed to determine volume.
It has also been known to weigh the collected fluid and to determine the volume from the weight. The weight and thus volume is determined in U.S. Pat. No. 3,583,400 of Menhardt by means for moving the collection fluid and determining the momentum of the fluid from which the mass of collected fluid is calculated. In an article by N. J. Randall, entitled "A New URO Flowmeter for Routine Clinical Use" in the January, 1975 issue of Biomedical Engineering (London), a simple strain gauge transducer system associated with a cantilever beam is employed to make direct weight measurements. A special tube is provided to absorb and redistribute urine flow into the container to prevent momentum during flow from effecting the weight measurement.
It is also known to use the weight measurements of collected fluid to control the fluid collection process itself. For instance, in U.S. Pat. No. 3,583,400 of Menhardt, a switch controls fluid flow in a collection container in resonse to measurement of collected fluid. Weight is used to stop a blood collection process when the amount collected exceeds a preselected weight in an apparatus shown in U.S. Pat. No. 3,977,567 of Rudd. In U.S. Pat. No. 4,390,073 of Rosen, blood in a collection bag is weighed to control collection.
While other approaches for work, the measurement of weight to determine volume is preferred. However, volumentric measurements based on weighing the collected fluid chamber can be no more accurate than the accuracy of the weight measurement itself. Therefore, it is important to make sure that it is only the weight of the fluid being measured and not the weight of the container or the forces associated with the collection system connected with the container which can affect the weight measurement. One of the advantages that weighing has over other methods of volume measurement is that the measurement does not require a special rigid container for level sensing and can be used after collection has commenced or after it is complete. In known systems which measure volume, however, because of the accuracy problem caused by variable forces applied by the upstream collection systems attached to the container, initial amounts of fluid collected in a bag upon catherization and before the bag is attached to the weighing apparatus are not measured in order to zero the weighing apparatus. This is done to partially offset some of the error introduced by the error producing forces applied by the force isolation system against the container.